Well known systems for bonding such restorative materials to teeth are usually composed of three components. Component 1 contains an acid (e.g. phosphoric acid or maleic acid) for partially etching the dental enamel and the dentine, component 2 contains hydrophilic crosslinkable compounds which flow well onto the hard dental substance, and component 3 contains crosslinkable substances which combine well with component 2 but are markedly more hydrophobic than dental enamel and dentine. Mostly, component 3 also contains one or more photoinitiators by means of which the adhesive system is hardened by irradiation with light. Such pretreatments for the adhesive bonding of hard dental substance to restorative materials are described, for example in U. Blunck, Quintessenz (1996), 47 (1), 19-35; R. Frankenberger, N. Kramer, J. Sindel, Dtsch. Zahnartzl. Z. (1996), 51, 556-560; and A. Pagliarini, R. Rubini, M. Rea, C. Campese, R. Grandini, Quintessence International (1996), 27, 265-270. As can be derived from these literature citations, dental surfaces pretreated in this way combine well with the corresponding dental materials and lead to a durable bond between restorative material and tooth.
In previous years, adhesive systems were also developed which unite components 2 and 3 in one component in order to reduce the overall work required for the fixing procedure, as described, e.g., in EP-A-0 234 934 and 0 305 083.
A disadvantage of the light-curing adhesive systems described is that the thickness of the fixing film that occurs can not be controlled accurately and is no longer flexible due to curing by means of irradiation. When inlays or onlays are used, this leads, for example, to an undesirable increase in the preparation and thus adversely affects the accuracy of fit to a considerable extent. As can be derived from the ISO standard 9917, even film thicknesses of the adhesive system of more than 25 .mu.m lead to accuracies of fit that are no longer acceptable, which means a considerable amount of extra grinding work for the dentist and, in unfavourable cases (e.g. due to breakage of an inlay during bite control), make it necessary for the dental technician's work to be done afresh.
In order to overcome these disadvantages, adhesive systems were developed which, instead of the components 2 or 3 described above, contain two partial components 2a and 2b and 3a and 3b respectively, which are mixed together before being applied to the hard dental substance (see, e.g., "Clearfil Line Bond 2" in Reality now, 1996, 80, 2). As a result of this mixing, a chemical curing process is started, which causes the entire adhesive system to cure within a few minutes. As a rule, however, the person carrying out the treatment still has sufficient time before curing is complete to apply the restorative material to the preparation in which the adhesive system is still sufficiently flexible to achieve a sufficient accuracy of fit. Any excess adhesive system can be pressed out at the edge of the preparation.
A disadvantage of these adhesive systems is their relatively labour-intensive application process since, compared with the radiation-curing systems described at the beginning, one more component and one more mixing stage is required (component 2 or 3 is composed of two compositions 2a and 2b and 3a and 3b respectively to be stored spatially separate from one another). This may easily lead to confusion on the part of the dentist and hence cause lasting damage to the adhesive power of the system. A further disadvantage of this adhesive system is the fact that the person carrying out the treatment is dependent on the setting time of the fixing system in question which can lead to unnecessary waiting times during the treatment or, in the case of more rapidly setting systems, may entail premature curing of the adhesive, with the same disadvantages as with light-curing systems.